Removal of iron scale is a problem in many operations within the household. Such operations includes cleaning toilet bowls, sinks, tubs, tiles, etc. Preparations such as sold under the trademark Lime-A-Way of Economics Laboratory, Inc., St. Louis, have been used for removing lime rust and scale. Formulations listed on the Lime-A-Way packages may be sulfamic acid; a combination of sulfamic and hydroxyacetic acids; phosphoric and citric acids; phosphoric and hydroxyacetic acids; and citric acid. There are also patents to acidid cleaning preparations. One such cleaning preparation is disclosed by Otrahalek in U.S. Pat. No. 4,032,466 which teaches a thickened acid cleaner concentrate composition consisting essentially of: a strong aqueous inorganic acid in an amount from about 5 to about 78 weight percent; an organic acid selected from the group consisting of oxalic, tartaric, citric acid and mixtures thereof in an amount from about 1 to about 6 weight percent; a onionic surfactant in an amount from about 7 to about 23 weight percent; an anionic surfactant in an amount from about 1 to about 7 weight percent; a flocculating agent containing bivalent iron, trivalent iron or trivalent aluminum in soluble form in acid solution and in an amount of from about 1 to about 12 weight percent; and water in an amount from about 75 to about 12 weight percent; all weight percents being based on the total composition. The nonionic surfactant and the anionic surfactant when taken together constitute from about 10 to about 30 weight percent of the composition and the nonionic surfactant constitutes from about 75 to about 90 weight percent of the combined weight percent of nonionic and anionic surfactants. This composition forms ferrous hydroxide flocks when the acidic cleaning solution is made basic by the addition of an alkaline agent. Wegst et al. in U.S. Pat. No. 3,173,875 describe acid detergent compositions for removing stains from porcelain-type fixtures. These compositions comprise an aqueous acid detergent solution of at least about 0.2 weight percent of an acid selected from the group consisting of phosphoric acid, hydrochloric acid and mixtures thereof and at least about 0.5 weight percentof a water-soluble ferrous salt selected from the group consisting of ferrous chloride, ferrous sulfate, ferrous nitrate and mixtures thereof. While such formulations are suitable, phosphates are ecologically unsound. Thus, there is a desire to avoid both phosphates and phosphoric acid to reduce the load on disposal and waste systems. Thompson, in U.S. Pat. No. 3,794,523, discloses scale removal by an emulsion having an acidic outer phase of an acid selected from hydrochloric, hydrofluoric, acetic, sulfamic and formic acids and mixtures thereof. Streicher in U.S. Pat. No. 2,973,191 discloses a method of substantially inhibiting the corrosiveness, in aqueous solution of a cleaning solution of at least one acid from the group consisting of formic, acetic, sulfamic and glycolic acids, towards stainless steels, which comprises incorporating into the solution a minor amount of a ferric ion-contributing salt, other than an iron halide.
Pfizer's Citrosolv process was developed to remove boiler deposits at elevated temperatures containing iron oxides, copper oxides and copper metal in a single solution. It ostensibly eliminates time-consuming intermediate draining and rinsing steps common to mineral-acid cleaning practices. Pfizer in data sheet 672 copyrighted 1981 describing the Citrosolv process states on page 5:
"In many cases of chemical cleaning of iron oxides it is desirable to add a chemical reducing agent to the citric formula, which donates electrons of its own, and therefore speeds up the process of converting ferric iron to the more readily soluble ferrous iron. The addition of a reducing agent to the citrate formula is recommended in cases where the impervious nature of the oxide scale acts as a barrier to penetration by the solvent to the substrate, or where the substrate is a lesser source of electrons than ordinary carbon steels (eg, passive stainless steels). Traditionally, bisulfite and thiourea have been used for this purpose. More recently, ascorbic and erythorbic acids.sup.2 have been used because of their nontoxic properties.
"Users of the Citrosolv Process .sup.1 noted that levels of dissolved iron increased slowly at the start of a typical boiler cleaning. The rate accelerated once a small critical level of ferrous ion had been dissolved into solution. Working from this observation, it was found that the addition of small amounts of ferrous salts (0.05%) to the fresh Citrosolv solution shortens the total time to perform the boiler cleaning."
The references referred to in the super-scripts are U.S. Pat. Nos. 3,072,502 (Alfano) and 3,248,269 (Bell), as specified on page 20 of the Pfizer data sheet. The first of these patents (Alfano) describes a process for removing copper-containing iron oxide scale from metal surfaces, which comprises contacting the surfaces with an aqueous solution containing at least about 1% w/v citric acid together with sufficient base to provide a pH between about 2.5 and about 5; the base being selected from the group consisting of ammonia, an unsubstituted ethanolamine and a water-soluble aliphatic hydrocarbon amine, continuing the contact with the iron content of the solution becomes substantially constant, subsequently adjusting the pH of the solution with one of the bases to a value between about 8 and about 10, and maintaining the pH within the alkaline range while continuing said contact until the copper content of said solution becomes substantially constant. The second of these patents (Bell) describes a method of removing copper-containing iron oxide scale from metal surfaces, consisting essentially of contacting the surfaces with a composition comprising a substantially neutral aqueous solution of ammonium citrate, continuing the contact until the iron content of the solution becomes substantially constant as the pH rises, then continuing the contact at a pH of at least 9 and maintaining the pH with ammonia until the copper content of the solution becomes substantially constant. The contacting method is carried out at elevated temperatures and pressures.
Notwithstanding the above quotation, neither Alfano nor Bell discloses the use of ferrous ion to remove rust. When Pfizer Chemical Division Technical Service Center was questioned on this issue, applicants were advised that Pfizer had not been able to show in the lab that addition of an iron reducing agent to citric acid speeds magnetite dissolution and that they are a bit puzzled by the statement in their literature. Although the Pfizer publication makes allusion that addition of ferrous ion to the citrate formulation speeds magnetite dissolution, the facts do not bear out the same.